Here we demonstrate the use of an advanced microfabrication technique, known as ultrafast laser inscription (ULI) with chemical etching, optimised for the fabrication of micro-optic systems in fused silica. ULI is a precision laser micromachining tool which relies on the high peak intensities associated with focused femtosecond pulses of light to locally modify the structure of a dielectric material. One manifestation of this modification is that the etch-rate of the modified regions can be increased by up to two orders of magnitude compared to that of pristine material, depending on the specific ULI parameters and the chemical etchant used. This capability means that ULI facilitates the repeatable fabrication of three-dimensional freeform structures in glass with micrometre resolution. Firstly, we present the results of investigations aimed at optimising the fabrication process and show that by controlling the laser polarisation during inscription, an etch-rate selectivity of 100 and a fivefold decrease in surface roughness can be achieved. We then demonstrate the characterisation of a microlens fabricated with optimum inscription parameters, including measurements of the lens surface profile, surface roughness and throughput, before demonstrating that the local surface roughness can be further decreased to below 5 nanometres by post-manufacture flame polishing.
The need for high speed wavefront sensing within astronomical adaptive optics is growing, especially when scaling existing systems to ELTs. A photonic lantern (PL) could be advantageous with such systems because the output can be formatted onto a fast 1D CCD array separated from the telescope focal plane. We investigate the coupling of light from the focal plane into a simple four mode PL via simulations within RSoft. The output intensity distribution of the single mode cores when the input wavefront is affected by tip or tilt is analysed and compared with a quad cell of detector pixels typically used for a Shack-Hartmann.
The advent of 30 m class Extremely Large Telescopes will require spectrographs of unprecedented spectral resolution in order to meet ambitious science goals, such as detecting Earth-like exoplanets via the radial velocity technique. The consequent increase in the size of the spectrograph makes it challenging to ensure their optimal environmental stabilization and precise spectral calibration. The multimode optical fibers used to transport light from the telescope focal plane to the separately housed environmentally stabilized spectrograph introduces modal noise. This phenomena manifests as variations in the light pattern at the output of the fiber as the input coupling and/or fiber position changes which degrades the spectrograph line profile, reducing the instrument precision. The photonic lantern is a guided wave transition that efficiently couples a multimode point spread function into an array of single modes. If arranged in a linear array at the input of the spectrograph these single modes can in principle provide a diffraction-limited mode noise free spectra in the dispersion axis. In this paper we describe the fabrication and throughput performance of the hybrid reformatter. This device combines the proven low-loss performance of a multicore fiber-based photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect that performs the reformatting function to a diffraction-limited pseudo-slit. The device provided an in laboratory throughput of 65 ± 2% at 1550 ± 20 nm and an on-sky throughput of 53 ± 4% at 1530 ± 80 nm using the CANARY adaptive optics system at the William Herschel Telescope.
Using a photonic reformatter to eliminate the effects of conventional modal noise could greatly improve the stability of a high resolution spectrograph. However the regimes where this advantage becomes clear are not yet defined. Here we will look at where modal noise becomes a problem in conventional high resolution spectroscopy and what impact photonic spectrographs could have. We will theoretically derive achievable radial velocity measurements to compare photonic instruments and conventional ones. We will discuss the theoretical and experimental investigations that will need to be undertaken to optimize and prove the photonic reformatting concept.
Ultrafast laser inscription is a versatile manufacturing technique which can be used to modify the refractive index of various glasses on a microscopic scale. This enables the production of a number of photonic devices such as waveguides, beam-splitters, photonic lanterns, and diffraction gratings. In this paper, we report on the use of ultrafast laser inscription to fabricate volume phase transmission gratings in mid-infrared transmitting chalcogenide glass.
We describe the optimisation of the laser inscription process parameters enhancing grating performances via the combination of spectrally resolved grating transmission measurements and theoretical analysis models. The first order diffraction efficiency of the gratings was measured at mid-infrared wavelengths (3-5 μm), and found to exceed 60% at the Littrow blaze wavelength, compared to a substrate external transmittance of 67%. This impressive result implies the diffraction efficiency should exceed 90% for a grating substrate treated with an anti-reflection coating. There is excellent agreement between the modelled grating efficiency and the measured data, and from a least squares fit to the measured data the refractive index modulation achieved during the inscription process is inferred. These encouraging initial results demonstrate that ultrafast laser inscription of chalcogenide glass may provide a potential new and alternative technology for the manufacture of astronomical diffraction gratings for use at near-infrared and mid-infrared wavelengths.
This paper reports on the modal noise characterisation of a hybrid reformatter. The device consists of a multicore-fibre photonic lantern and an ultrafast laser-inscribed slit reformatter. It operates around 1550 nm and supports 92 modes. Photonic lanterns transform a multimode signal into an array of single-mode signals, and thus combine the high coupling efficiency of multimode fibres with the diffraction-limited performance of single-mode fibres. This paper presents experimental measurements of the device point spread function properties under different coupling conditions, and its throughput behaviour at high spectral resolution. The device demonstrates excellent scrambling but its point spread function is not completely stable. Mode field diameter and mode bary-centre position at the device output vary as the multicore fibre is agitated due to the fabrication imperfections.
Due to their high efficiency and broad operational bandwidths, volume phase holographic gratings (VPHGs) are often
the grating technology of choice for astronomical instruments, but current VPHGs exhibit a number of drawbacks
including limits on their size, function and durability due to the manufacturing process. VPHGs are also generally made
using a dichromated gelatine substrate, which exhibits reduced transmission at wavelengths longer than ~2.2 μm,
limiting their ability to operate further into the mid-infrared.
An emerging alternative method of manufacturing volume gratings is ultrafast laser inscription (ULI). This technique
uses focused ultrashort laser pulses to induce a localised refractive index modification inside the bulk of a substrate
material. We have recently demonstrated that ULI can be used to create volume gratings for operation in the visible,
near-infrared and mid-infrared regions by inscribing volume gratings in a chalcogenide glass. The direct-write nature of
ULI may then facilitate the fabrication of gratings which are not restricted in terms of their size and grating profile, as is
currently the case with gelatine based VPHGs.
In this paper, we present our work on the manufacture of volume gratings in gallium lanthanum sulphide (GLS)
chalcogenide glass. The gratings are aimed at efficient operation at wavelengths around 1 μm, and the effect of applying
an anti-reflection coating to the substrate to reduce Fresnel reflections is studied.
Spectroscopy is a technique of paramount importance to astronomy, as it enables the chemical composition, distances
and velocities of celestial objects to be determined. As the diameter of a ground-based telescope increases, the pointspread-
function (PSF) becomes increasingly degraded due to atmospheric seeing. A degraded PSF requires a larger
spectrograph slit-width for efficient coupling and current spectrographs for large telescopes are already on the metre
scale. This presents numerous issues in terms of manufacturability, cost and stability.
As proposed in 2010 by Bland-Hawthorn et al, one approach which may help to improve spectrograph stability
is a guided wave transition, known as a “photonic-lantern”. These devices enable the low-loss reformatting of a
multimode PSF into a diffraction-limited source (in one direction). This pseudo-slit can then be used as the input to a
traditional spectrograph operating at the diffraction limit. In essence, this approach may enable the use of diffractionlimited
spectrographs on large telescopes without an unacceptable reduction in throughput.
We have recently demonstrated that ultrafast laser inscription can be used to realize “integrated” photoniclanterns,
by directly writing three-dimensional optical waveguide structures inside a glass substrate. This paper presents
our work on developing ultrafast laser inscribed devices capable of reformatting a multimode telescope PSF into a
It is possible to significantly improve the performance of astronomical spectroscopy by taking the Point Spread Function from a near diffraction-limited telescope and reformatting it using photonic technologies. This can improve the stability of a conventional instrument or provide an interface to single mode instruments developed for the telecommunications industry. We compare different options for reformatting and interfacing with different types of instruments and examine them using set metrics. We then examine the relative merits for instruments that could be developed for astronomy.